Ultra high molecular weight ethylene polymers refer to those having a viscosimetrically determined molecular weight of greater than 1×106 g/mol. Because of their extraordinary properties, such as higher abrasion resistance and low sliding friction, such polymers have a multitude of uses. Consequently, they are used in materials handling, bulk materials handling, as well as in medical applications such as joint sockets in prosthetic joints.
High and very high molecular weight ethylene polymers refer to those having a viscosimetrically determined molecular weight of greater than that of a conventional high-density polyethylene, for example, greater than about 400,000 g/mol, and less than that of an ultra high molecular weight polyethylene. Such polymers have good impact strength and abrasion resistance, although somewhat lower than that of ultra high molecular weight ethylene polymers. They are melt processible and are used, for example, in food handling cutting boards and ortheses.
Because of these novel properties, the processing of high to ultra high molecular weight polyethylene is highly complex. Ram-extrusion and compression molding of powdered raw materials are processes used to produce molded parts, whereby the molded parts manufactured often still exhibit the characteristics of the raw powder. Films and fibers are produced using solution or gel processes, which require large amounts of solvents. An objective is therefore to develop new high to ultra high molecular weight polyethylenes, which have improved processability.
According to the present state of technology, ultra high molecular weight polyethylene is manufactured according to the low pressure process using heterogeneous Ziegler catalysts. Such catalysts are, for example, described in the following patent documents: EP186995, DE3833445, EP575840 and U.S. Pat. No. 6,559,249.
Other known catalysts for olefin polymerization are single site catalysts. According to the present state of technology, ultra high molecular weight polymers are manufactured using these catalysts only in exceptional cases and under economically unprofitable conditions. Consequently, heterogeneous constrained-geometry catalysts form ultra high molecular weight polyethylene only with moderate activity and increased long chain branching, which can lead to reduced hardness and worse abrasion properties. With so-called phenoxy-imine catalysts, UHMWPE is obtained only at low activity at economically unprofitable temperature levels. Examples of these as well as other metallocenes are described in WO9719959, WO0155231, Adv. Synth. Catal 2002, 344, 477-493, EP0798306, as well as in EP0643078.
While some bridged metallocene catalysts are known, the catalysts have not been taught in a heterogeneous polymerization process in such a way as to achieve high to ultra high molecular weight in production of ethylene homopolymers and copolymers comprising predominantly ethylene monomers. Examples of such catalysts are described in U.S. Pat. Nos. 6,417,302; 7,005,398; 7,109,278; and 7,169,864.
Surprisingly, bridged single site catalysts with a suitable ligand structure were found in connection with the present invention, which when optionally used with aluminoxanes as co-catalysts, not only permitted the manufacture of ultra high molecular weight polyethylenes with a viscosimetrically determined molecular weight of at least about 0.7×106 g/mol, but also produced products with improved processability. The reason for the improved processability, without being bound to a theory, has to do with the narrower molecular weight distribution Mw/Mn of 2 to 6 compared to polymers that were manufactured using Ziegler catalysts and have a molecular weight distribution Mw/Mn of 3 to 30.